Conventionally, in order to improve erosion resistance of an electrode portion of a spark plug, an electrode tip formed from a noble metal, such as platinum, iridium, ruthenium, or rhodium, or from a noble metal alloy is provided on a distal end portion of a ground electrode and on a front end portion of a center electrode (see Japanese Patent Application Laid-Open (kokai) No. 2006-32185, hereinafter referred to as Patent Document 1). Such electrode tips are usually laser-welded to the center electrode and to the ground electrode.
Generally, metals used to form the center electrode and the ground electrode differ from those used to form the electrode tips. Accordingly, the center electrode and the ground electrode differ in thermal expansion coefficient from the electrode tips. Thus, in a certain welded condition in which the electrode tip is welded to the center electrode or to the ground electrode, the electrode tip may separate from the center electrode or from the ground electrode, for example, in a heat load test in which heating and cooling are repeated, or in the course of operation of an internal combustion engine.